package com.wc.AlgoOJ_homework.AlgoOJ_HomeWork1.S_重复字符串;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.StringTokenizer;

/**
 * @Author congge
 * @Date 2024/1/20 15:46
 * @description http://43.138.190.70:8888/p/LQ1124
 */
public class Main {
    static FastReader sc = new FastReader();
    static PrintWriter out = new PrintWriter(System.out);
    static int k;
    static int N = 100010;
    // leNum[i][j] 表示第i个字母为j的数量
    static int[][] letNum = new int[N][128];
    static String s;

    public static void main(String[] args) {
        k = sc.nextInt();
        s = sc.next();
        char[] chs = s.toCharArray();
        if (chs.length % k > 0) {
            out.println(-1);
        } else {
            int ans = 0;
            int lets = chs.length / k;
            for (int i = 0; i < chs.length; i += lets) {
                for (int j = 0; j < lets; j++) {
                    letNum[j][chs[i + j]]++;
                }
            }
            // 一行里面那个字母最多，我们就改成他
            for (int i = 0; i < lets; i++) {
                int maxN = 0;
                for (int j = 'a'; j <= 'z'; j++) {
                    maxN = Math.max(letNum[i][j], maxN);
                }
                ans += k - maxN;
            }
            out.println(ans);
        }
        out.flush();
    }
}

class FastReader {
    StringTokenizer st;
    BufferedReader br;

    FastReader() {
        br = new BufferedReader(new InputStreamReader(System.in));
    }

    String next() {
        while (st == null || !st.hasMoreElements()) {
            try {
                st = new StringTokenizer(br.readLine());
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        return st.nextToken();
    }

    int nextInt() {
        return Integer.parseInt(next());
    }

    String nextLine() {
        String s = "";
        try {
            s = br.readLine();
        } catch (IOException e) {
            e.printStackTrace();
        }
        return s;
    }

    long nextLong() {
        return Long.parseLong(next());
    }

    double nextDouble() {
        return Double.parseDouble(next());
    }

    // 是否由下一个
    boolean hasNext() {
        while (st == null || !st.hasMoreTokens()) {
            try {
                String line = br.readLine();
                if (line == null)
                    return false;
                st = new StringTokenizer(line);
            } catch (IOException e) {
                throw new RuntimeException(e);
            }
        }
        return true;
    }
}